During dissimilatory sulfate reduction or sulfur oxidation, adenylylsulfate (APS) reductase catalyzes reversibly the two-electron reduction of APS to sulfite and AMP [1]. Found in several bacterial lineages and in Archaeoglobales, APS reductase is a heterodimer composed of an alpha subunit containing a noncovalently bound FAD, and a beta subunit containing two [4Fe-4S] clusters [1]. Described by this model is the alpha subunit of APS reductase, sharing common evolutionary origin with fumarate reductase/succinate dehydrogenase flavoproteins.